REVIEW
http://dx.doi.org/10.5653/cerm.2012.39.4.127
pISSN 2233-8233 · eISSN 2233-8241
Clin Exp Reprod Med 2012;39(4):127-131
Transcription factors in the maintenance and survival
of primordial follicles
Eun-Jin Lim, Youngsok Choi
Department of Biomedical Science, CHA University, Seoul, Korea
Primordial follicles are formed prenatally in mammalian ovaries, and at birth they are fated to be activated to primary follicles, to be dormant,
or to die. During the early stage of folliclulogenesis, the oocyte undergoes dynamic alterations in expression of numerous genes, which are regulated by transcription factors. Several germ-cell specific transcriptional regulators are critical for formation and maintenance of follicles. These
transcriptional regulators include: Figla, Lhx8, Nobox, Sohlh1, and Sohlh2. A subset of these transcriptional regulators is mutated in women with
ovarian insufficiency and infertility. Establishment of this oocyte pool is essential for fertility. This review focuses on these transcriptional regulators of female primordial follicles.
Keywords: Primordial follicles; Transcription factors; Figla; Lhx8; Nobox; Sohlh1; Sohlh2
Introduction
This is an Open Access article distributed under the terms of the Creative Commons Attribution
Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits
unrestricted non-commercial use, distribution, and reproduction in any medium, provided the
original work is properly cited.
ovulation, when ootidogenesis continues to produce a released egg.
In the final stage of oogenesis, the egg develops into the ovum, which
is a mature egg cell. In humans and other mammals, the secondary
oocyte becomes an ovum soon after fertilization with a sperm (Figure 1).
Oogenesis occurs with folliculogenesis in mammals. Folliculogenesis is a complex process that depends on numerous factors including
both extra-ovarian and intra-ovarian factors. Early folliculogenesis
proceeds during embryonic development. Primordial germ cells
(PGCs) are the origin of oocytes. PGCs appear in the endoderm near
the yolk sac at the third to fourth weeks of gestation in the human
and then migrate to the primitive gonad along the hindgut and dorsal mesentery by the 7th week of gestation [1]. After the arrival of
PGCs in the gonad, the PGCs proliferate rapidly and then their numbers reach their maximum of up to 6 to 7 million at the 20th week of
gestation. The PGCs enter meiosis and become primary oocytes from
the 15th week of gestation [2]. Meiosis in primary oocytes is arrested
at prophase I around birth. During meiosis, the primary oocyte becomes a primordial follicle by individual lapping with surrounding
pregranulosa cells. Once primordial follicles are formed, they have
three possible fates. Some of them are recruited for growth and differentiation to be ovulated, which is accompanied by the synthesis
of a unique set of proteins in both germ cells and surrounding somatic cells. Most of them remain dormant during the ovarian cycle.
Copyright © 2012. THE KOREAN SOCIETY FOR REPRODUCTIVE MEDICINE
www.eCERM.org
Oogenesis, a type of gametogenesis, is the production of ova or
egg cells in females. Oogenesis occurs in all sexually reproductive
species, and it consists of different stages of ovum production. There
are several stages in ovum maturation in mammals, namely, the oogonium, primary oocyte, secondary oocyte, ootid, and ovum. In the
first stage of oogenesis, the oogonium undergoes oocytogenesis,
generating a primary oocyte through meiosis. Like the oogonium,
the diploid primary oocyte contains two complete sets of chromosomes (4n). During meiosis, the primary oocyte produces a haploid
secondary oocyte (2n). This process is halted halfway through until
Received: Dec 17, 2012 ∙ Revised: Dec 26, 2012 ∙ Accepted: Dec 26, 2012
Corresponding author: Youngsok Choi
Department of Biomedical Science, CHA University, 566 Nonhyeon-ro,
Gangnam-gu, Seoul 135-913, Korea
Tel: +82-2-3468-3504 Fax: +82-2-3468-3446 E-mail: [email protected]
*This work was supported by the Priority Research Centers Program through the
National Research Foundation of Korea (NRF) funded by the Ministry of Education,
Science and Technology (2009-0093821), and by the Korea Healthcare Technology
R&D Project, Ministry for Health, Welfare & Family Affairs, Republic of Korea
(A084923).
127
Clin Exp Reprod Med 2012;39(4):127-131
Meiosis: Oogonia→ Oocyte
Atresia
Do
rm
an
t, s
ur
viv
al
During ovarian life
Activation
Primordial follicle
Follicle
Primary follicle growing
Ovulation
Figure 1. Regulatory networks of transcriptional regulators, SOHLH1,
SOHLH2, NOBOX, LHX8 and FIGLA (Modified from Choi and Rajkovic
[5]). Black line, direct regulation; Blue line, putative regulation; ZP,
zona pellucida; GDF9, growth differentiation factor 9; POU5f1, POU
class 5 homeobox 1; PAD6, peptidyl arginine deiminase type VI; OOG1,
oogenesin 1; cMOS, moloney sarcoma oncogene; ZAR1, zygote arrest 1; OOSP1, oocyte secreted protein 1; NPM2, nucleoplasmin 2;
DMRT1, doublesex and mab-3 related transcription factor 1; STRA8,
stimulated by retinoic acid gene 8; CASP, caspase; BAX, BCL2-associated X protein; KITL, kit ligand; TDRD, tudor domain containing; OAS1,
2’-5’ oligoadenylate synthetase 1; EXO1, exonuclease 1; NLRP, NLR
family, pyrin domain containing; DPPA3, developmental pluripotency-associated 3; H1foo, H1 histone family; member O, oocyte-specific.
Among them, some undergo the apoptotic pathway during follicle
growth. Development of high quality oocytes during folliculogenesis
is highly critical for proper fertilization and development because it
affects early embryonic survival, the establishment and maintenance
of pregnancy, and fetal development [3,4]. Primordial follicles are the
smallest follicles in the mammalian ovary. During follicular development, the primary oocyte in the primordial follicle undergoes numerous germ-cell specific transcriptions which are crucial for the follicle‘s
maintenance and survival until the end of the ovarian cycle. However, the mechanism(s) of maintenance and survival of primordial follicles remains unknown. Recent studies have described several germ
cell-specific transcription factors in the ovary including factor in the
germline alpha (FIGLA), newborn ovary homeobox protein (NOBOX),
LIM-homeobox protein 8 (LHX8), spermatogenesis and oogenesisspecific basic helix-loop-helix transcription factor 1 (SOHLH1), and
SOHLH2 [5]. The knockout mouse models show premature ovarian
128
failure (POF). This review is focused on these germ-cell specific transcriptional factors, which might be crucial for understanding of the
regulatory mechanism of maintenance and survival of primary oocytes until ovulation.
Factor in the germline alpha (FLGLA)
FIGLA is one of the well-known germ cell-specific transcriptional
factors. FIGLA expresses in the oocyte as early as embryonic day 14.5
(E14.5) [6]. FIGLA contains a basic helix-loop-helix (bHLH) domain
which binds to a specific DNA binding element, E-box (CANNTG). FIGLA is involved in regulation of numerous germ-cell specific genes
including zona pellucida protein 1 (Zp1), Zp2, and Zp3 through an Ebox motif (CANNTG) [6]. The E-boxes on the promoter of Zp1, Zp2,
and Zp3 are conserved among species [7]. The physiological function
of FIGLA was revealed by using a gene knockout mouse model. Figla
deficient mice are sterile due to a defect of the primordial follicles in
the ovary. Figla deficient mice lose all of the primordial follicles right
after birth [8]. A recent study that performed whole gene profile analysis of the Figla deficient ovary revealed more downstream target
genes including Nlrp family members (Nlrp4a, Nlrp4b, Nlrp4f, Nlrp5
[Mater], Nlrp14), Pou5f1, Kit, Exo1, Zbed3, Dppa3, Oas1h, and Padi6 [9].
Interestingly, Figla deficiency shows a sexually dimorphic phenotype
because Figla null males are fertile with normal testicular morphology [8]. The ectopic expression of Figla represses testis specific genes,
such as a Tdrd family member 6 (Tdrd6), Mvh, and Mili [10]. These findings suggest that Figla activates numerous oocyte-specific genes
and represses testis-specific genes at the same time [9,10] and that
the dimorphic regulation of FIGLA is crucial for the formation and
maintenance of primordial follicles. A recent study showed that a
mutation of FIGLA, the deletion c.419-421 delACA (p.140 delN), was
found in patients with POF [11]. The deletion c.419-421 delACA disrupted the binding affinity of FIGLA to TCF3 [11]. TCF3 is known to be
one of the FIGLA interacting factors [6,12].
Newborn ovary homeobox gene (NOBOX)
Nobox encodes a transcription factor containing a homeobox domain as a DNA binding site [13]. NOBOX belongs to a group of tissuespecific homeobox genes and may play an important role in oogenesis and ovarian development. Nobox is preferentially expressed during mammalian folliculogenesis. Nobox can be first detected in the
oogonia as early as E15.5 and then remains abundant in oocytes
throughout folliculogenesis [13,14]. A deficiency in Nobox shows a
sexually dimorphic phenotype like that of Figla null mice. The gene
knockout males are fertile and normal, but null females are infertile.
Nobox null ovaries appear morphologically normal, containing pri
http://dx.doi.org/10.5653/cerm.2012.39.4.127
EJ Lim and Y Choi Transcriptional network in primordial follicles
mordial follicles [14]. However, Nobox null ovaries lost follicles rapidly
after birth and impaired the transition of primordial follicles to primary follicles [14]. In microarray analysis, numerous germ cell-specific genes are regulated in the newborn ovary from Nobox null mice
including Gdf9, Bmp15, Pou5f1, Zar1, c-Mos, Oog1, Pad6, Oosp1, Oas1d,
Oas1d, Oas1e, and Oas1h. These are regulated either directly or indirectly. NOBOX transregulates its target genes through NOBOX binding elements (NBE; TAA/GTTG/A) [15]. NOBOX appears to directly bind
to Gdf9, Pou5f1, and Pad6 through the NBE(s) present on their promoters [15,16]. In addition, one of the maternal genes, Npm2, contains one NBE which is required for basal transcriptional activity in
oocytes [17]. Interestingly, one of the male-determining genes, Dmrt1,
is overexpressed in the null ovaries. This suggests that NOBOX regulates sexually dimorphically during gametogenesis like FIGLA. A missense mutation, p.Arg355His, was found in the NOBOX homeobox
domain in a POF patient [18]. The mutation disrupted the NOBOX affinity to targeting DNA binding elements and had a dominant negative effect on the binding of wild-type NOBOX to DNA.
LIM-homeobox protein 8 (LHX8)
LHX8 is a member of the LIM-homeobox transcription factor family.
LHX8 contains two LIM domains and one homeobox domain that
are highly conserved with 93% identity between mice and humans.
LHX8 plays an important role in tissue patterning and differentiation
including that of neural tissues [19-21]. In the ovary, Lhx8 transcripts
localize to oocytes in germ cell clusters and primordial, primary, and
antral follicles in the mouse ovary [22]. Lhx8 is detectable as early as
E13.5 [22]. A deficiency of Lhx8 leads to female infertility [7,22]. Lhx8
null mice are infertile due to the absence of oocytes and impairment
of the transition of primordial follicles into primary follicles [22]. At
birth, histological examination shows that Lhx8-deficient ovaries are
grossly similar to newborn wild-type ovaries. However, Lhx8 null ovaries fail to maintain the primordial follicles after birth. This phenotype
is similar to that in Nobox deficient mice. The oocytes in primordial
follicles of the Lhx8 null ovary have a problem with the transition from
primordial to growing follicles and survival of the follicles. They exist
until postnatal day 7. This is shorter than the survival in Nobox deficient mice. An Lhx8 deficiency causes the misexpression of many
genes, such as Gdf9, Pou5f1, Nobox, Kit, and Kitl [22]. The comparison
of genes regulated in Lhx8 null and Nobox null newborn ovaries revealed a common set of genes including the Nlrp family members,
such as Nlrp14, Nlrp4c, and Nlrp4f [14,22,23]. These factors are also
down-regulated in Figla null mice [9]. The function of Nlrp family mem­
bers in the oocyte remains unclear. In addition, Stra8 is drastically upregulated in both Lhx8 and Nobox null newborn ovaries. Stra8 is detectable in the ovary between E12.5 and E16.5 during embryo develwww.eCERM.org
opment and then its expression is not detectable in the ovary [24].
Stra8 is crucial for the initiation of meiosis with premeiotic DNA replication, meiotic chromosome condensation, cohesion, synapsis, and
recombination [25]. This suggests that the repression of the meiotic
factor or the activation of other factors including Pou5f1, Gdf9, and
Nlrp family members by either NOBOX or LHX8 may be important for
the maintenance and survival of primordial follicles after birth.
Spermatogenesis and oogenesis specific basic
helix-loop-helix protein 1 (SOHLH1) and SOHLH2
SOHLH1 and SOHLH2 are bHLH germ cell-specific transcription factors in the development of both the ovary and testis. The expression
patterns of SOHLH1 and SOHLH2 are different from that of FIGLA,
NOBOX, and LHX8. In the ovary, the expression of Sohlh1 and Sohlh2
are confined to the germ cell, primordial follicles, and primary follicles, but not in secondary and growing follicles [7,26]. This indicates
that SOHLH1 and SOHLH2 may play important and unique roles in
primordial follicle formation, activation, and survival.
Ovaries of Sohlh1 deficient mice are completely devoid of follicles
by 3 weeks after birth [7]. During early development, most oocytes in
the 7-day-old Sohlh1 deficient ovaries are still enveloped by flat somatic cells similar to primordial follicles but now also contain multiple empty follicles in the ovary [7]. The defect in primordial follicle
maintenance and survival may result from the surrounding pre-granulosa cells. The disruption of Sohlh1 misregulates many germ-cell
specific genes. Among those, Lhx8 is regulated directly by SOHLH1
[7]. The phenotype of Sohlh2 deficiency has been shown to be very
similar to that of Sohlh1 knockout [26,27]. Sohlh2 null ovaries form
primordial follicles, but the primordial follicles limit growth and do
not differentiate from surrounding granulosa cells into cuboidal and
multilayered structures [26,27]. Sohlh2 deficient ovaries rapidly lose
the follicles after birth, with few remaining by 14 days of postnatal
life. The knockout mice ovaries misexpress numerous germ cell- and
oocyte-specific genes, including Sohlh1, Nobox, Figla, Gdf9, Pou5f1,
Zp1, Zp3, Kit, Oosp1, Nlrp14, H1foo, and Stra8. SOHLH2 may work together with SOHLH1. SOHLH1 and SOHLH2 can form heterodimer
[27]. In addition, they coordinate germ cell development by the regulation of Kit expression [28-30].
Conclusion
The molecular mechanisms of maintenance and survival of primordial follicles during ovarian cycles remain unclear. New insights from
recent studies of a series of germ-cell specific transcriptional regulators indicate that complex regulatory networks are involved in follicle
formation and maintenance in the ovary. A knockout mouse model
129
shows that they do not share redundancy among factors, but a unique
pathway. Analyzing their downstream target genes might elucidate
more mechanisms and pathways for developing a better understanding of the mechanism of activation and survival of ovarian follicles.
The regulatory networks of primordial follicles will give us understanding ovary-specific pathways and clues to regulate and treat female
fertility like premature ovarian failure.
Clin Exp Reprod Med 2012;39(4):127-131
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15.
Conflict of interest
No potential conflict of interest relevant to this article was reported.
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